1. Field of the Invention
This invention relates to a color image forming apparatus which forms a toner image on an image retainer by an electrophotographic system and transfers the toner image onto a transfer material to obtain the image.
2. Description of the Prior Art
An image forming method for obtaining a color image by use of an electrophotographic system is disclosed in Japanese Patent Laid-Open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985, 158475/1985, etc., for example. As described in these references, the image forming method repeats the formation of a latent image and a development on an image retainer in the number corresponding to the color separation number of an original image to superpose the color toner images on the image retainer and then transfer the resulting image to obtain the color image.
Color image forming apparatus to which the image forming method which repeats the formation and development of the latent images in accordance with the color separation number of the original image read by a color scanner onto the image retainer can be classified into the following groups. The first example of the apparatus disposes exposure devices and developing devices in the number corresponding to the color separation number (e.g. three colors of yellow, magenta, cyan or four colors including black in addition to these three colors) around a belt-shaped image retainer. The second example disposes the exposure devices and developing devices in the number corrresponding to the color separation number around a drum-shaped image retainer. Among them, the color image forming apparatus equipped with the belt-shaped imager retainer will be hereinafter explained.
In the color image forming apparatus, a belt-shaped image retainer, which is obtained by coating or depositing a photoconductor onto a flexible belt, and is brought into pressure contact with tension rollers, is rotated while keeping a sliding contact state with a guide member as the reference by the pressure contact, and is transferred so that its surface is kept always at a constant position. Image forming means consisting of a charging devices, exposure devices and a plurality of developing devices storing therein different color toners (yellow, magenta, cyan, black) is disposed around the belt-shaped image retainer with a predetermined gap with respect to the belt-shaped image retainer that is rotating.
In the color image forming apparatus, it would be possible to form registration marks on the image retainer, for example, to detect the registration marks by a sensor and to start sequentially the exposure of a plurality of the exposure devices on the basis of detection so as to start the formation of the latent image from the same position on the image retainer.
In the color image forming apparatus described above, however, even if the timing at which the exposure is started is determined by reading the registration marks formed on the belt-shaped image retainer by a single or a plurality of sensors, this method is not free from the requirement that the gap between the sensor(s) and the exposure devices and the arrangement gap between a plurality of exposure devices must be positioned with strict mechanical accuracy (about .+-.0.01-0.1 mm), but positioning of the gap between the sensor(s) and the exposure devices and the arrangement gaps between the exposure devices with strict mechanical accuracy is practically very difficult, That is, the mechanical accuracy conventionally obtained is up to about .+-.0.3-0.5 mm. If any exposure devices are removed for maintenance or the like, it is extremely difficult to position them once again to the same position.
In the color image forming apparatus described above wherein the toner images are superposed on the image retainer to form the color toner image, there is a problem that quality of the color toner image drops unless the exposure start position of a plurality of exposure devices onto the belt-shaped image retainer is set to a deviation below about 80 .mu.m per pixel, for example. Particularly when the belt-shaped image retainer is used, positioning control is more difficult than in the case of the drum-shaped image retainer.